The present invention relates in general to physical ion deposition devices, and in particular to a new and useful hollow cathode discharge vapor source for providing high melting temperature metal vapors for ion plating the inside surfaces of cylinders.
Physical vapor deposition (PVD) is a process in which a substance, usually a metal, is deposited onto the surface of an object called the substrate. The two most common forms of PVD are evaporation and sputtering. In the former case, the metal vapor is produced by evaporation of the material to be deposited. This process is carried out at low pressures (less than 10.sup.-5 Torr) and produces a line of sight coverage on the substrate causing shadowed areas not to be covered. The material to be evaporated must have a melting point low enough to be attained by resistive heating in an evaporation boat. For materials having a higher melting temperature, the latter form of PVD must be used. In this case, the metal vapor is produced by inert gas ions striking a target of the plating material and knocking (sputtering) atoms from its surface. This process takes place at higher pressures (10.sup.-3 Torr) which eliminates much of the shadowing present in evaporation, but has the disadvantage of having very low deposition rates, typically 100 A/min.
In the last decade, a third form of PVD called Ion Plating has been developed and characterized. PVD is known as ion plating when a negative bias is applied to the substrate during deposition which is carried out in the presence of an inert gas such as Argon. The negative bias ionizes the vapor of deposition material and inert gas, thereby producing an acceleration of these ions toward the substrate. This action sputter cleans the substrate during deposition as well as physically mixes the substrate material with the deposited material. When the deposition rate is greater than the sputtering rate, a film is formed having adhesion properties far superior to those produced by the evaporation or sputtering methods. Ion plating is further characterized by a high pressure operation (10.sup.-1 -10.sup.-2 Torr) which, because of the short mean free paths of the ions in the gas, provides excellent coating uniformity for irregularly shaped substrates. The high pressure operation of ion plating requires a deposition source capable of high metal vapor pressures.
The most commonly used vapor deposition source consists of a tantalum or tungsten boat which is resistively heated. The evaporation boat works well with low melting temperature metals but has several drawbacks. The first is that the melted metal may alloy with the boat, requiring frequent boat replacement. Another drawback is that the hot boat produces large amounts of heat which cannot be shielded without seriously affecting the ability to produce high metal vapor pressures. This heat requires the substrate to be placed at a considerable distance from the boat, reducing the coating efficiency. In addition, the boat must be connected to massive stationary electrical contacts necessary to carry the large currents needed (several hundred amperes), making the evaporation boat an essentially planar device.
When the material to be evaporated has a high melting temperature, an electron beam gun is used as the vapor source. The e-beam gun consists of an electron emitting filament, a magnet and a water cooled crucible. In operation, a current is run through the filament which is held at a potential of several thousand volts below the crucible. The emitted electrons are accelerated by this potential, bent and focussed by the magnet, and then strike the evaporant in the crucible causing it to melt and vaporize. The e-beam gun has an advantage over the evaporation boat in that less heat is generated, which allows the substrate to be closer to the vaporization source. Its major drawback is that the large filament potentials limit its use to pressures below 10.sup.-4 Torr. This makes it necessary to construct a complicated differential pumping system for use at the higher pressures required for ion plating. Consequently, its use is limited to a planar configuration as was the case with the evaporation boat.
Such conventional ion plating sources such as the electron-beam gun or the evaporation boat work well with geometries which are essentially planar. A large three dimensional object can be ion plated using multiple sources or a mechanism to rotate the substrate during deposition. These methods fail when trying to apply a uniform ion plated coating on the inside surfaces of tubes having a large length to inside diameter ratio.
The need for coating the inside of tubes has recently resulted in a system for sputtering in cylindrical coordinates. This system consists of a rod of the deposition material inserted coaxially inside a tube. Application of a potential of several thousand volts to the rod causes it to be sputtered, with the resulting atoms deposited on the substrate. This method has two major drawbacks. The deposition rate is very slow, with approximately 24 hours of continuous operation necessary to produce a reasonably thick coating (10.mu. meters), and it is believed that the material is sputter deposited rather than ion plated, and therefore would not have the good adhesion properties of an ion plated film.